The study aims to increase understanding of the determinants of retinochoroiditis in children with congenital toxoplasmosis and the consequences for visual outcomes. In addition, it will provide information on the developmental and neurological prognosis of congenital toxoplasmic retinochoroiditis and determine the effect of treatment on these outcomes. The analyses will be based on a prospective European multicenter cohort study conducted in 13 centers in 6 countries and a separate Danish cohort study, which involved standardized ophthalmic, pediatric, and parental assessments up to 4 and 6 years. In addition, the Danish study involved clinician assessments of intelligence quotient (IQ) and development at 6 years of age. Of the 277 infected, and 850 uninfected children enrolled, follow up data are available on 65% to 80% depending on outcome). These are the first prospective, population-based studies to have evaluated the long term risk of retinochoroiditis and its association with visual outcomes, and the first comparative studies of development, behavior, and neurologic impairment in infected and uninfected children. Analyses of children with congenital toxoplasmosis will examine how age at detection of retinochoroiditis is associated with the gestational age at infection, and hence fetal maturity. Associations with prenatal and postnatal treatment, intracranial lesions, and markers of disseminated infection will also be investigated. The study will examine associations between the age at detection and site of retinochoroiditis and visual outcomes at age 4 and 6. Methodological approaches for handling the interval censored covariate for gestation at infection will be evaluated as part of the study. Finally, to assess prognosis, scores for development, behavior, IQ, and parental concerns will be compared in infected children, with and without retinochoroiditis, and uninfected children. The study will provide etiologic information on the determinants of retinochoroiditis and its association with fetal development. The results will also provide clinical information on the long-term risk of retinochoroiditis, the need for follow up, the possible benefits and harms of treatment, and the consequences for visual function and development.